Many different types of platforms have been provided for drilling of oil wells offshore. In shallow waters, ridged platforms are most often used. In deeper water spar platforms may be more suitable. A spar platform is a floating vessel that is held at the location by anchor lines. Spar platforms typically have a long vertical cylindrical hull that supports a platform above the water line. When the platform provides space for drilling and maintaining oil or gas wells, the production wells may be provided along the outside edge of the platform or the production wells may be located in the center of the platform with a "moon bay" through the center of the spar.
Designs for spar platforms must consider vortex induced vibrations of the spar hull and may, depending on the prevailing currents and the dimensions and stiffness of the spar, utilize some method to suppress vibrations caused by vortex shedding. Typically, helical strakes have been proposed to suppress vortex induced vibrations. Helical strakes are commonly included on smoke stacks and other vertical cylinders in air to reduce vibrations caused by vortex shedding, but the effectiveness of helical strakes in water, and on shorter cylinders such as spars, is less than what is desired.
A variety of shrouds and fairings have been proposed or used to reduce vibrations caused by shedding of vortices from tubulars in water. These shrouds and fairings may be considerably more effective in reducing vibrations caused by vortex shedding than helical strakes, but are typically difficult and expensive to provide on a tubular as large as the hull of a spar platform.
It is therefore an object of the present invention to provide a spar platform that is less susceptible to vortex induced vibrations than prior art spar platforms. It is a further object to provide such a platform having, over at least a portion of its axial length, a perforated shroud. It is another object to provide such a platform wherein the perforated shroud can be easily installed and is relatively inexpensive.